Evaluation of factors influencing replication of serotype 1 Marek's disease vaccines in the chicken lung

Factors influencing replication of serotype 1 Marek's disease vaccines in the lung of chickens within the first 10 days of age (doa) were evaluated. In particular, the effect of vaccine efficacy, age/route of vaccination, and vaccine dose were examined in three experiments. In the first experiment, three vaccine pairs, each pair consisting of a high protective (HP) and a low protective (LP) vaccine (CVI988/BP5 and CVI988-Clone C, 648A80 and 648A100, R2 and R2/23) were used to inoculate chickens subcutaneously (s.c.) with 2000 plaque-forming units (PFU) at hatch. DNA load in the lung was significantly higher in the HP vaccine group than the LP vaccine group at 5 and 10 doa in two of the three vaccine pairs. In the lung, at 5 doa, early MDV gene transcripts (ICP4 and pp38) were detected in most vaccine groups, whereas late MDV gene transcripts (gB and gI) were detected only in the HP vaccine group. In the second experiment, chickens were vaccinated in ovo or s.c. at hatch with 2000 PFU R2, R2/23, or CVI988/BP5. Compared with s.c. vaccination, in ovo vaccination resulted in higher MDV DNA load in the lung at 3 doa, lower or equal MDV DNA load at 5 doa, and lower MDV DNA load at 10 doa. In the third experiment, chickens were vaccinated s.c. at hatch with either 2000 or 10,000 PFU strain R2. There were no statistically significant differences in the load of MDV DNA in the lung after vaccination with R2 between the two doses. Our results showed that HP vaccines tend to replicate better than LP vaccines in the lung; and vaccine replication in the lung within the first 10 days of age was affected by the age/route of inoculation (in ovo versus s.c.) but not by the dose of vaccine administered.